This application is based on Application No.2001-196363, field in Japan on Jun. 28, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
This invention relates to an air/fuel ratio control apparatus for an internal combustion engine, and more specifically, to an air/fuel ratio control apparatus for an internal combustion engine that removes poisonous components present in an exhaust gas using a catalytic purifying device provided in an exhaust system path.
2. Related Background Art
Conventionally, there has been well known an air/fuel ratio control apparatus for an internal combustion engine that has an exhaust gas purifying function of removing poisonous components, such as NOx, HC, and CO, emitted from the internal combustion engine. To achieve this function, a three-way catalyst for removing such poisonous components in an exhaust gas is provided in an exhaust system of the internal combustion engine.
If a fuel cut is performed, however, a large amount of oxygen is absorbed by the three-way catalyst, so that even if the fuel cut is reset afterward, the air/fuel ratio does not immediately return to the state before the fuel cut is started. This causes an excess oxygen state, in which an NOx reduction action becomes inactive and the three-way catalyst cannot fully exert its effect. As a result, there is the danger of poisonous components being discharged into the air.
In view of this problem, a technique of suppressing the amount of NOx discharged after the reset of the fuel cut is disclosed in Japanese Patent Application Laid-open No. Hei 8-193537 (hereinafter, the xe2x80x9cfirst prior artxe2x80x9d). With this technique, the amount of oxygen absorbed by a three-way catalyst during a fuel cut is obtained based on an intake air amount or a period of time of the fuel cut. When the fuel cut is reset, an air/fuel ratio is controlled to be on a rich side close to a theoretical air/fuel ratio for a very short time period according to the amount of the absorbed oxygen. In this manner, the amount of NOx discharged after the reset of a fuel cut is suppressed.
Another technique of solving the stated problem is disclosed in Japanese Patent Application Laid-open No. Hei 11-280457 (hereinafter, the xe2x80x9csecond prior artxe2x80x9d). With this technique, the amount of oxygen absorbed by a three-way catalyst during a fuel cut is obtained based on an intake air amount or period of the fuel cut. When the fuel cut is reset, an air/fuel ratio is enriched in a step-by-step manner using an initial value corresponding to the amount of the absorbed oxygen, and then the enriched air/fuel ratio is brought back to an theoretical air/fuel ratio at a predetermined speed, thereby suppressing the amount of NOx discharged after the reset of the fuel cut. Also, after the enrich processing is temporarily suspended by acceleration, it is judged whether the three-way catalyst returns to the state before the fuel cut according to the output of an oxygen concentration sensor arranged downstream of the catalyst. If it is judged that the catalyst returns to the original state, re-enrich processing is not performed.
Even in the case of a fuel cut where the same amount of oxygen is supplied according to the amount of oxygen absorbed by a catalyst before the fuel cut, there may occur a phenomenon where the amount of oxygen absorbed varies after the fuel cut and therefore the deterioration degree of an NOx purification rate changes. For instance, if the oxygen absorption amount of a catalyst before a fuel cut is small and the catalyst is in a rich state, this may result in a situation where the fuel cut does not increase the amount of absorbed oxygen to a level where the NOx purification rate is decreased. Conversely, if the catalyst oxygen absorption amount before a fuel cut is large and the catalyst is in a lean state, this may cause a situation where the NOx purification rate is significantly decreased even by a fuel cut performed for a short time.
As described above, there are variations in the amount of oxygen absorbed by a catalyst before fuel cuts and therefore there occur variations in the oxygen absorption amount of the catalyst when the fuel cuts are reset. This creates the necessity to appropriately perform enrich processing according to the oxygen absorption amounts of the catalyst after the fuel cuts.
In the first and second prior arts, however, enrich processing is performed only according to the amount of oxygen supplied during a fuel cut and the oxygen absorption state of a catalyst after the fuel cut is not taken into account during the enrich processing. This causes variations in the catalyst oxygen absorption amount after the rich processing. As a result, there may occur a problem that too enriched catalyst reduces a THC purification rate, or a problem that enrich processing is not sufficiently performed and the Nox purification rate does not return to an adequate level.
Also, there has been recently devised a catalyst system that increases a purification efficiency by providing two three-way catalysts (hereinafter respectively referred to as a xe2x80x9cfront three-way catalystxe2x80x9d and a xe2x80x9crear three-way catalystxe2x80x9d), with the rear three-way catalyst being arranged at a position downstream of the front three-way catalyst. In this system, poisonous components in an exhaust gas are purified as much as possible (about 90% or more of the components are removed, for instance) by the front three-way catalyst and the poisonous gas that is not purified by the front three-way catalyst is purified by the rear three-way catalyst with reliability. This makes it possible to raise the exhaust gas purification rate to around 100% in total using the front and rear three-way catalysts. To achieve this high purification rate, it is required to always maintain both of the front and rear three-way catalysts in conditions where optimal exhaust purification capacities are obtained.
If a fuel cut is performed in such a catalyst system where a front three-way catalyst and a rear three-way catalyst are provided, however, a phenomenon may occur where there is a difference in oxygen absorption amount between the front and rear three-way catalysts.
If most of oxygen supplied by a fuel cut is absorbed by a front three-way catalyst, for instance, a rear three-way catalyst hardly absorbs oxygen, so that it is sufficient that enrich processing is performed only for the front three-way catalyst after the fuel cut is reset.
Also, if a large amount of oxygen exceeding the absorption capacity of a front three-way catalyst is supplied by a fuel cut, the oxygen absorption amount of a rear three-way catalyst is also increased, so that it becomes necessary to perform enrich processing for both of the front and rear three-way catalysts after the fuel cut is reset.
Such a variation in the amount of oxygen supplied to a rear three-way catalyst by a fuel cut is caused by the oxygen absorption state of a front three-way catalyst during the fuel cut.
Even in the case of the fuel cut, as described above, where the same amount of oxygen is supplied, the oxygen absorption state of the front three-way catalyst before the fuel cut causes a variation in the oxygen absorption state of the front three-way catalyst during the fuel cut, so that the supply of oxygen to the rear three-way catalyst is also affected.
That is, to accurately detect the amount of oxygen absorbed by a rear three-way catalyst during a fuel cut, it is required to detect the oxygen absorption amount of a front three-way catalyst during the fuel cut and to estimate the amount of oxygen flowing to the rear three-way catalyst.
In the first and second prior arts, however, enrich processing is performed only according to the amount of oxygen supplied by a fuel cut and therefore the oxygen absorption amounts of a front three-way catalyst before and during the fuel cut are not taken into account during the enrich processing. This causes variations in the oxygen absorption amounts of the front and rear three-way catalysts after the rich processing. As a result, there may occur a problem that the front or rear three-way catalyst becomes too rich and therefore the THC purification rate is reduced or a problem that enrich processing is not sufficiently performed and the Nox purification rate does not return to an adequate level.
Also, to remove oxygen absorbed by a rear three-way catalyst due to a fuel cut, it is required to supply a rich air/fuel mixture to the rear three-way catalyst. Therefore, during the removal of oxygen absorbed by the rear three-way catalyst, a front three-way catalyst needs to be shifted from an optimal state to a rich state to reduce THC and CO purification rates. In this case, rich exhaust gas including large amounts of THC and CO is supplied, so that oxygen absorbed by the rear three-way catalyst at a position downstream of the front three-way catalyst is removed and THC and CO that were not purified by the front three-way catalyst are consumed by the removal of oxygen from the rear three-way catalyst. As a result, there is no fear of these poisonous components passing through the rear three-way catalyst and being discharged into the air.
Also, although enriching a front three-way catalyst with respect to an optimal state reduces THC and CO purification rates, it becomes possible to maintain a high NOx purification rate, to prevent NOx from flowing to a rear three-way catalyst whose NOx purification rate is reduced by oxygen absorption, and to prevent NOx from being discharged into the air.
However, objects of the first and second prior arts are to bring the oxygen absorption amount of a catalyst back to an optimal state by performing enrich processing after the reset of a fuel cut. Therefore, these prior arts do not contain a concept that a catalyst is enriched with respect to an optimal oxygen absorption state to remove oxygen absorbed by another catalyst arranged downstream. Therefore, there is a problem that it is difficult to remove oxygen absorbed by a rear three-way catalyst.
The present invention has been made to solve the stated problems and an object of the present invention is to achieve an air/fuel ratio control apparatus for an internal combustion engine where the purification ability of a three-way catalyst is brought back to an optimal state by expelling oxygen absorbed by the three-way catalyst due to a fuel cut immediately after the fuel cut without being affected by the oxygen absorption amount of the three-way catalyst before the fuel cut.
According to the present invention, there is provided an air/fuel ratio control apparatus for an internal combustion engine comprising: a three-way catalyst that is provided in an exhaust system of the internal combustion engine and removes poisonous substances present in an exhaust gas; an oxygen concentration sensor that detects an oxygen concentration of the exhaust gas passed through the three-way catalyst; a fuel injection valve that injects fuel into the internal combustion engine; an injection amount adjusting means for adjusting an amount of the fuel injected by the fuel injection valve so that an air/fuel ratio becomes a predetermined air/fuel ratio; an fuel injection suspending means for suspending the fuel injection by the fuel injection valve on a predetermined condition concerning an operational state of the internal combustion engine; and a correction factor setting means for setting an air/fuel ratio enrich correction factor according to a difference between an output voltage of the oxygen concentration sensor and a predetermined target voltage set for the oxygen concentration sensor, wherein, during a predetermined time after a fuel cut is reset by the fuel injection suspending means, the injection amount adjusting means performs air/fuel ratio enrich processing by adjusting the amount of the fuel injected by the fuel injection valve according to the air/fuel ratio enrich correction factor set by the correction factor setting means.
There is also provided an air/fuel ratio control apparatus, wherein the three-way catalyst is a front three-way catalyst, and the apparatus further comprises: a rear three-way catalyst that is provided downstream of the oxygen concentration sensor and removes poisonous substances present in the exhaust gas passed through the front three-way catalyst; an intake air amount detecting means for detecting an amount of intake air into the internal combustion engine; and an oxygen absorption amount estimating means for estimating an oxygen absorption amount of the rear three-way catalyst resulting from the fuel cut, according to the intake air amount detected by the intake air amount detecting means and the output voltage generated by the oxygen concentration sensor during the fuel cut by the fuel injection suspending means, wherein The correction factor setting means changes the target voltage set for the oxygen concentration sensor according to the oxygen absorption amount of the rear three-way catalyst estimated by the oxygen absorption amount estimating means.